5-(Hydroxymethyl)furfural (HMF) is a versatile intermediate that can be obtained in high yield from biomass sources such as naturally occurring carbohydrates, including fructose, glucose, sucrose, and starch. Specifically, HMF is a conversion product of hexoses with 6 carbon atoms. It is known that HMF can be oxidized using a variety of reagents to form any of four different products, which can themselves be converted to one or more of the others:

The selective oxidation of an alcohol functionality in the presence of an aldehyde functionality on the same compound is difficult because of the high reactivity of the aldehyde group. Furthermore, if HMF is reacted with molecular oxygen (O2), the aldehyde functionality would be expected to oxidize more rapidly than the alcohol and the expected product would be predominantly 5-(hydroxymethyl)furan-2-carboxylic acid (Sheldon, R. A. and Kochi, J. K. “Metal Catalyzed Oxidations of Organic Compounds”, Academic Press, New York, N.Y. 1981, p 19).
Diformylfuran (DFF) has been prepared from HMF using CrO3 and K2Cr2O7 (L. Cottier et al., Org. Prep. Proced. Int. (1995), 27(5), 564; JP 54009260) but these methods are expensive and results in large amounts of inorganic salts as waste. Heterogeneous catalysis using vanadium compounds has also been used, but the catalysts have shown low turnover numbers (DE 19615878, Moreau, C. et al., Stud. Surf. Sci. Catal. (1997), 108, 399-406). Catalytic oxidation has been demonstrated using hydrogen peroxide (M. P. J. Van Deurzen, Carbohydrate Chem. (1997), 16(3), 299) and dinitrogen tetraoxide (JP 55049368) which are expensive. The relatively inexpensive molecular oxygen (O2) has been used with a Pt/C catalyst (U.S. Pat. No. 4,977,283) to form both DFF and furan-2,5-dicarboxlic acid (FDA), but yielded low amounts of DFF. Good yields were found for FDA, but only as the disodium salt which resulted in wasteful salt formation during conversion to the acid form.
Metal bromide catalysts have been used to oxidize substituted alkylbenzenes to various products including the oxidation of alkyl to aldehydes, alkyl to alcohols, alkyl to acids, alcohol to acid, and aldehydes to acids (W. Partenheimer, Catalysis Today, 23(2), 69-158, (1995)). However, in such cases, the aldehyde product is either a minor component or is quickly oxidized further. FDA has also been prepared using a Co/Mn/Br catalyst from 5-methylfurfural with DFF seen as a minor byproduct (V. A. Slavinskaya, et al., React. Kinet. Catal. Lett. (1979), 11(3), 215-20).
DFF has been polymerized to form polypinacols and polyvinyls (Cooke, et al., Macromolecules 1991, 24, 1404). However, preparation of polyesters prepared from diformylfuran is not known in the literature.
DFF can also be used to produce unsubstituted furan. Unsubstituted furan is an important commodity in the chemical industry used in the production of tetrahydrofuran. Supported metal catalysts have been used in the decarbonylation of the monoaldehyde furfural to furan, but a basic promoter is required, adding expense and complexity to the process (U.S. Pat. No. 3,007,941, U.S. Pat. No. 4,780,552).
Considering the aforementioned discussion, there is a need for an inexpensive, high yield process for the preparation of both DFF and FDA that does not produce large amounts of waste products and which lends itself to easy separation and purification. Additionally, there is a need for a high yielding process to prepare unsubstituted furan from relatively inexpensive, renewable sources.